A drilling rig comprises a hydraulic rotary percussive hammer drill sliding on a drilling jar and driving one or more drill rods, the last of these rods bearing a tool known as a bit which is in contact with the rock. Such a hammer drill generally is intended to bore rather deep holes so that explosive charges can be laid therein. The hammer drill is therefore the main element which, on the one hand, imparts rotation to the bit and imparts percussion via the drill rods so as to penetrate the rock, and, on the other hand, supplies an injection fluid to extract the debris from the bored hole.
A hammer drill comprises a mechanism, driven by one or more flows of hydraulic fluid from a main supply circuit of the percussion mechanism, acting on the drill rods via a shank which is able to pass on, on the one hand, the successive blows brought about by a percussion piston and, on the other hand, the rotation due to a rotary hydraulic motor.
The force with which the hammer drill bears against the drill rods, and therefore, by transmission, with which the bit bears against the rock, is obtained using the hydraulic motor of the drilling jar. More specifically, the bearing force is transmitted from the body of the hammer drill to the shank via an abutment element incorporated into the hammer drill. This abutment element may consist of a fixed rotary friction part, but more generally, in the case of high-power hammer drills, consists of an abutment piston one surface of which is hydraulically supplied so as to transmit the bearing force through a fluid.
European patent applications EP 0 058 650 and EP 0 856 637 disclose abutment piston devices for which the hydraulic supply comes from the main supply circuit of the percussion mechanism. However, when the operator shuts off this main supply circuit and he activates, for example, only the rotary motor, the surface of the abutment piston is no longer hydraulically supplied and the piston can therefore come into direct contact with the body of the hammer drill, something which may give rise to considerable damage.